Enhancements to a dual column nitrogen producing cryogenic air separation unit

ABSTRACT

Enhancements to a dual column, nitrogen producing cryogenic air separation unit are provided. Such enhancements include an improved air separation cycle that uses three condenser-reboilers and recycles a portion of the vapor from one or more of the condenser-reboilers to the incoming feed stream and or the compressed purified air streams to yield improvements in the performance of such dual column, nitrogen producing cryogenic air separation units in terms of overall nitrogen recovery as well as power consumption.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. provisional patent application Ser. No. 63/029,909 filed May 26, 2020 the disclosure of which is incorporated by reference.

TECHNICAL FIELD

The present inventions relates to enhancements to a dual column, nitrogen producing cryogenic air separation unit, and more particularly to improvements in the performance of such dual column, nitrogen producing air separation units in terms of overall nitrogen recovery as well as power consumption. The performance improvements are generally attributable to an enhanced air separation cycle that uses three condenser-reboilers and recycles a portion of the vapor from one or more of the condenser-reboilers to the incoming feed stream and or the compressed, purified air streams.

BACKGROUND

Industrial gas customers often seek nitrogen product slates at volumes and pressures that typically require very large cryogenic air separation units. Such large scale or high volume nitrogen producing air separation units often use a dual distillation column arrangement, including a higher pressure column and a lower pressure column in which gaseous nitrogen products are withdrawn from the distillation columns at relatively high pressures or at two different pressures. In the conventional dual column nitrogen producing air separation unit, the higher pressure column and lower pressure column are thermally linked in a heat transfer relationship by a main condenser, which liquefies a portion of the nitrogen-enriched vapor from the overhead of the higher pressure column to be used as reflux to the higher pressure column. Supplemental refrigeration for such conventional nitrogen producing air separation cycles is typically provided via an upper column turbine arrangement. An example of a large volume nitrogen producing air separation unit is disclosed in U.S. Pat. No. 4,453,957.

Over the course of the past several decades numerous improvements to such large volume nitrogen producing cryogenic air separation units have been developed to address shortcomings in the performance of such large-scale nitrogen producing air separation cycles.

For example, U.S. Pat. No. 5,098,457 discloses a double distillation column arrangement for large volume nitrogen production where the main condenser is not driven by reboiling a portion of the lower pressure bottoms liquid, but rather the main condenser is driven by a portion of the kettle liquid from the higher pressure column. More specifically, U.S. Pat. No. 5,098,457 discloses a split kettle arrangement wherein a portion of the kettle liquid from the higher pressure column is re-boiled in the main condenser and another portion of the kettle liquid from the higher pressure column is directed to an intermediate location on the lower pressure column.

U.S. Pat. No. 6,330,812 discloses another double distillation column arrangement for large volume nitrogen production that employs three condenser-reboilers including a double main condenser configuration where both main condensers are driven by reboiling kettle liquid from the higher pressure column while the third condenser-reboiler associated with the lower pressure column is driven by the oxygen-enriched liquid taken from the bottom of the lower pressure column.

Finally, U.S. Pat. No. 6,257,019 discloses a triple distillation column arrangement for large volume nitrogen production. In addition to the conventional lower pressure distillation column and higher pressure distillation column each with a separate condenser-reboiler, the triple distillation column arrangement also utilizes an intermediate pressure distillation column and a third condenser operatively associated with the intermediate pressure distillation column. The triple distillation column arrangement is believed to demonstrate very high nitrogen recoveries at comparatively lower power consumption levels. However, a key disadvantage to the triple distillation column arrangement is the higher capital costs associated with the additional column, the third condenser/reboiler, and additional compressors needed for the intermediate pressure column feed.

Other improvements to such large volume nitrogen producing air separation units have been employed in applications requiring a portion of the nitrogen to be provided as liquid nitrogen. In applications where is no need or desire to make a liquid nitrogen product from the air separation unit, the upper column turbine arrangement disclosed in U.S. Pat. No. 4,453,957 is adequate. However, in end-user applications where a liquid nitrogen product is required or desired, employing the conventional upper column turbine arrangement is economically impractical, as the arrangement leads to high liquefaction unit power costs and unworkable rangeability requirements. Such previous improvements included arrangements that employ a lower column turbine arrangement, a waste gas expansion arrangement, a nitrogen product expansion and recycle arrangement, and a warm recycle turbine refrigeration arrangement.

What is needed are further enhancements to such large-scale nitrogen producing cryogenic air separation units to improve nitrogen recovery and/or reduce the associated operating costs (i.e. power costs) over the above-identified prior art systems and previously disclosed improvements thereto.

SUMMARY OF THE INVENTION

The present invention may be characterized as an air separation unit comprising: (a) a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; (b) an adsorption based pre-purifier unit configured for removing impurities from the compressed air stream and producing a compressed, purified air stream; (c) a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; and (d) a distillation column system comprises a higher pressure column and a lower pressure column linked in a heat transfer relationship via at least three condenser-reboilers. The distillation column system produces a lower pressure nitrogen product stream, a medium/high pressure nitrogen product stream, a waste stream and a recycle that is a portion of the vapor from one or more of the condenser-reboilers that is recycled to the incoming feed air stream and or the compressed, purified air stream.

The higher pressure column is configured to receive the cooled, compressed, purified air stream and produce a nitrogen enriched overhead and an oxygen-enriched kettle stream while the lower pressure column is configured and produce a lower pressure nitrogen product stream, an overhead stream and an oxygen-enriched bottoms. The first main condenser-reboiler of the three condenser-reboilers is configured to condense a first portion of the nitrogen enriched overhead from the higher pressure column against the oxygen-enriched bottoms from the lower pressure column to produce a nitrogen reflux stream for the higher pressure column and an ascending vapor stream in the lower pressure column from the boil-off of the oxygen-enriched bottoms. The second condenser-reboiler is operatively associated with the higher pressure column and configured to condense a second portion of the nitrogen enriched overhead from the higher pressure column against a first split portion of the oxygen-enriched kettle stream from the higher pressure column to produce a liquid nitrogen stream and a recycle stream from the boil-off of the oxygen-enriched kettle stream. The third condenser-reboiler is operatively associated with the lower pressure column and configured to condense the nitrogen overhead from the lower pressure column against the oxygen bottoms from the lower pressure column to produce a nitrogen reflux stream for the lower pressure column and a waste stream. In addition, a second split portion of the oxygen-enriched kettle stream is introduced into the lower pressure column at an intermediate location while a third portion of the nitrogen enriched overhead from the higher pressure column is taken as a medium/high nitrogen product stream.

In some embodiments, the recycle stream is compressed in a recycle compressor and recycled back to the main air compression system, preferably to an inter-stage location of the main air compressor while in other embodiments the recycle stream is compressed in a recycle compressor and recycled back to and/or combined with the compressed, pre-purified air stream. Still other embodiments contemplate the recycle compressor configured as a cold compressor driven by a booster loaded turbine configured to expand a diverted portion of the medium/high nitrogen product stream to produce an exhaust stream from the booster loaded turbine that is combined with the lower pressure nitrogen product stream. In all embodiments, refrigeration is preferably supplied to the air separation unit by use of an upper column turbine arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it may be better understood when taken in connection with the accompanying drawings in which:

FIG. 1 is a schematic process flow diagram of an embodiment of a dual column, nitrogen producing cryogenic air separation unit in accordance with an embodiment of the present invention;

FIG. 2 is a schematic process flow diagram of another embodiment of the present dual column, nitrogen producing cryogenic air separation unit; and

FIG. 3 is a schematic process flow diagram of yet another embodiment of the present dual column, nitrogen producing cryogenic air separation unit.

DETAILED DESCRIPTION

As discussed in more detail below, the disclosed cryogenic air separation systems and methods provide certain performance enhancements to large-scale, dual column, nitrogen producing cryogenic air separation units targeted to increase nitrogen recovery and reduce power consumption compared to prior art large-scale, dual column, nitrogen producing cryogenic air separation units.

Turning to FIG. 1, there is shown a schematic illustration of the large volume, nitrogen producing cryogenic air separation unit 10. In a broad sense, the depicted air separation unit includes a main feed air compression train or system, a turbine air circuit, a main heat exchange system, and a distillation column system.

In the main feed compression train shown in FIG. 1, the incoming feed air 22 is typically drawn through an air suction filter house and is compressed in a multi-stage, intercooled main air compressor arrangement 24 to a pressure that can be between about 6.5 bar(a) and about 11 bar(a). This main air compressor arrangement 24 may include integrally geared compressor stages or a direct drive compressor stages, arranged in series or in parallel. The compressed air stream 26 exiting the main air compressor arrangement 24 is fed to a pre-purification unit 28 to remove impurities including high boiling contaminants. The pre-purification unit 28, as is well known in the art, typically contains two beds of alumina and/or molecular sieve operating in accordance with a temperature swing adsorption cycle in which moisture and other impurities, such as carbon dioxide, water vapor and hydrocarbons, are adsorbed. One or more additional layers of catalysts and adsorbents may be included in the pre-purification unit 28 to remove other impurities such as carbon monoxide, carbon dioxide and hydrogen to produce the compressed, purified air stream 29. Particulates may be removed from the feed air in a dust filter disposed upstream or downstream of the pre-purification unit 28.

As shown in FIG. 1, the compressed, purified air stream 29 may be split into a plurality of air streams, including a turbine air stream 31 and a compressed, purified feed air stream 33. Turbine air stream 31 may be further compressed in a turbine air booster compressor 37 and subsequently cooled in an aftercooler 39 to form a boosted pressure turbine air stream which is then partially directed to the main heat exchange system which includes heat exchanger 52A where it is partially cooled. The partially cooled, boosted pressure turbine air stream 38 exits heat exchanger 52A and is expanded in turbine 35 to produce exhaust stream 64 that is directed to lower pressure column 74. In this manner, a portion of the refrigeration for the air separation unit 10 is thus provided by the expansion of the turbine air stream 38 in turbine 35. The remaining portion of the compressed, purified feed air stream 33 is fully cooled in heat exchangers 52A and 52B and exits the cold end of heat exchanger 52B as a fully cooled air stream 47. The fully cooled air stream 47 is introduced into higher pressure column 72 at a location proximate the bottom of the higher pressure column 72.

Cooling the compressed, purified feed air stream 33 and partially cooling the boosted pressure turbine air stream in the heat exchangers 52A and 52B is preferably accomplished by way of indirect heat exchange with the warming streams which include the medium/high pressure nitrogen product stream 105, the lower pressure nitrogen product stream 110 and a recycle stream 100 from the distillation column system to produce cooled air streams suitable for rectification in the distillation column system.

The heat exchangers 52A and 52B are preferably brazed aluminum plate-fin type heat exchangers. Such heat exchangers are advantageous due to their compact design, high heat transfer rates and their ability to process multiple streams. They are manufactured as fully brazed and welded pressure vessels. For larger air separation units handling higher flows, the heat exchanger may be constructed from several cores which must be generally connected in series as illustrated in the drawings.

The turbine based refrigeration circuit used in cryogenic air separation units are often referred to as either a lower column turbine (LCT) arrangement or an upper column turbine (UCT) arrangement which are used to provide refrigeration to a cryogenic air distillation column systems. In the UCT arrangements shown in FIGS. 1-3, the boosted turbine air stream is preferably at a pressure in the range from between about 6 bar(a) to about 10.7 bar(a) and partially cooled to a temperature in a range of between about 140 K and about 220 K. This cooled, compressed turbine air stream that is introduced into the turbine to produce an expanded, cold exhaust stream 64 that is then introduced into the lower pressure column of the distillation column system. The supplemental refrigeration created by the expansion of the turbine air stream is thus imparted directly to the lower pressure column thereby alleviating some of the cooling duty of the main heat exchanger. In some embodiments, the turbine may be coupled with a compressor, either directly or by appropriate gearing.

While the turbine based refrigeration circuit illustrated in the FIGS. 1-3 is shown as an upper column turbine (UCT) circuit where the turbine exhaust stream 64 is directed to the lower pressure column, it is contemplated that the turbine based refrigeration circuit alternatively may be a lower column turbine (LCT) circuit or a partial lower column (PLCT) where the expanded exhaust stream is fed to the higher pressure column of the distillation column system.

The illustrated distillation column system includes a higher pressure column 72, a lower pressure column 74, a first main condenser-reboiler 75, a second condenser-reboiler 85 and a third condenser-reboiler 95. The higher pressure column 72 typically operates in the range from between about 7 bar(a) to about 12 bar(a) whereas lower pressure column 74 operates at pressures between about 4.5 bar(a) to about 7 bar(a). Cooled feed air stream 47 is preferably a vapor air stream slightly above its dew point, although it may be at or slightly below its dew point, that is fed into the higher pressure column 72 for rectification resulting from mass transfer between an ascending vapor phase and a descending liquid phase that is initiated by a nitrogen based reflux stream. This separation process within the higher pressure column 72 produces a nitrogen-rich column overhead 89 and crude oxygen-enriched bottoms liquid also known as kettle liquid 80 which is taken as kettle stream 88.

The higher pressure column 72 and the lower pressure column 74 are preferably linked in a heat transfer relationship via the first main condenser-reboiler 75 wherein a first portion 73 of the nitrogen-rich vapor column overhead extracted from the higher pressure column 72 is condensed within the first main condenser-reboiler 75 shown as a once-through heat exchanger being located in the base of lower pressure column 74 against the oxygen-rich liquid column bottoms 77 residing in the bottom of the lower pressure column 74. The boiling of oxygen-rich liquid column bottoms 77 initiates the formation of an ascending vapor phase within lower pressure column 74. The condensation produces a liquid nitrogen stream 81 that is used to reflux the lower pressure column 74 to initiate the formation of descending liquid phase therein. If desired, a portion of the reflux stream may be withdrawn as liquid product.

A second portion 83 of the nitrogen-rich vapor column overhead extracted from the higher pressure column 72 is condensed within the second condenser-reboiler 85 shown as a once-through heat exchanger disposed in a separate condenser vessel 84. The second condenser-reboiler 85 is operatively associated with the higher pressure column 72 and configured to condense the second portion 83 of the nitrogen enriched overhead from the higher pressure column 72 against a subcooled first split portion 86 of the oxygen-enriched kettle stream 88 from the higher pressure column 72 to produce a liquid nitrogen stream 82 and a recycle stream 100 from the boil-off of the oxygen-enriched kettle stream. Liquid nitrogen stream 82 could be added to the liquid nitrogen reflux stream 81 that is used to reflux the lower pressure column 74.

The remaining portion of the oxygen-enriched kettle stream, referred to as the second split portion 87, is subcooled and then flashed via valve 187 and introduced into an intermediate location of the lower pressure column 74, a number of stages above the first main condenser-reboiler 75. In addition, a third portion of the nitrogen-rich vapor column overhead extracted from the higher pressure column 72 which is not liquefied in either of the first main condenser-reboiler or the second condenser-reboiler but is withdrawn as a medium pressure or high pressure nitrogen product stream 105 and warmed in heat exchangers 52A and 52B to produce a warmed medium/high pressure nitrogen product stream 115.

In the lower pressure column 74, the ascending vapor phase includes the boil-off from the first main condenser-reboiler 75 as well as the exhaust stream 64 from the upper column turbine 35 introduced at an intermediate location of the lower pressure column 74. The descending liquid is initiated by nitrogen reflux stream 96 from the third condenser reboiler 95 which is released into the lower pressure column 74.

Lower pressure column 74 is also provided with a plurality of mass transfer contacting elements, that can be trays or structured packing or other known elements in the art of cryogenic air separation. The separation occurring within lower pressure column 74 produces a nitrogen overhead 92, a lower pressure nitrogen product stream 110 taken from a location proximate an upper section of the lower pressure column several stages below the overhead 92, and an oxygen-rich liquid column bottoms 77. The lower pressure nitrogen product stream 110 is further warmed in heat exchangers 52B, 52A to produce a lower pressure, warmed nitrogen product stream 120.

As indicated above, the third condenser-reboiler 95 is associated with the lower pressure column 74 and disposed in a vessel 94. The third condenser-reboiler 95 is configured to condense the nitrogen overhead 92 from the lower pressure column 74 against the portion of the oxygen bottoms liquid 77 that is not reboiled. That portion of the oxygen bottoms liquid 77 from the lower pressure column 74 is subcooled and the subcooled stream 176 is flashed via valve 177 with the resulting stream 178 into the boiling side of the third condenser-reboiler 95. The condensed liquid produced by the third condenser-reboiler 95 is nitrogen reflux stream 96 used to reflux the lower pressure column 74 while the vapor generated is withdrawn as waste stream 93 which is warmed in heat exchanger 52B and the warmed waste stream 193 may be utilized to regenerate the pre-purifier unit 28.

The recycle stream 100 is taken from the vapor stream exiting the second condenser-reboiler 85 and is preferably recycled back to the main air compression system and combined with the incoming feed air stream 22. As shown in FIG. 1, the heat exchangers 52A and 52B are configured to extract refrigeration from the recycle stream 100 and cool the compressed, purified air in part via indirect heat exchange with the recycle stream 100. The warmed recycle stream 102 is then introduced to the main air compressor 24, preferably at an inter-stage location, or optionally compressed in a recycle compressor (not shown) and cooled in an aftercooler (not shown) prior to combining with the feed air stream.

Turning now to FIG. 2, there is shown partial schematic diagrams of an alternate embodiment of the present system and method. Many of the features, components and streams associated with the nitrogen producing air separation unit shown in FIG. 2 are similar or identical to those described above with reference to the embodiment of FIG. 1 and for sake of brevity will not be repeated here. The key differences between the nitrogen producing air separation units illustrated in FIG. 2 compared to those in the corresponding arrangement shown in FIG. 1 is the recycle stream is compressed in recycle compressor 104 and cooled in aftercooler 103 and returned to a location downstream of the pre-purifier but upstream of the main heat exchange system and combined with the compressed, pre-purified air stream 29.

In the embodiment of FIG. 2, the recycle compressor 104 may need to be sized and operated to fully compress the recycle stream to pressures needed to combine with compressed, pre-purified air stream whereas the embodiment of FIG. 1, the recycle pressure exiting an optional recycle compressor may be lower as the recycle stream will be further compressed in the main air compressor. Advantageously, the pre-purifier unit 28 in FIG. 2 is likely smaller and less costly than the pre-purifier unit in FIG. 1 due to the increase in volume of incoming feed air due to the addition of the recycle stream upstream of pre-purifier unit 28 in FIG. 1 but downstream od the pre-purifier unit in the embodiment of FIG. 2. Also, by avoiding the pre-purifier unit, the recycle stream of FIG. 2 does not bear the associated pressure drop.

The embodiment depicted in FIG. 3 shows a further arrangement of the large-scale, dual column, nitrogen producing air separation unit where the recycle compressor 202 is a cold compressor configured to compress the cold recycle stream 200. The cold compressed recycle stream 208 is cooled in heat exchanger 52B and sent to the higher pressure column 72. In the illustrated embodiment, the cold compressor 202 is driven by a booster loaded turbine 205. The booster loaded turbine 205 is configured to expand a diverted portion 207 of the medium/high nitrogen product stream 105 with the exhaust stream 206 from the booster loaded turbine 205 being returned to the lower pressure nitrogen product stream 110. Again, as many of the features, components and streams associated with the nitrogen producing air separation unit shown in FIG. 3 are similar or identical to those described above with reference to the embodiment of FIG. 1 the associated descriptions will not be repeated here.

Examples

When compared against the various existing prior art air separation cycles for large scale dual column, nitrogen producing air separation units, the use of split kettle arrangement with three condenser-reboilers and a portion of the vapor from the second condenser-reboiler being recycled as contemplated by the present systems and methods generally reduces power consumption of the air separation unit by about 5.0% or more while concurrently increasing nitrogen recovery. Computer model simulations of a prior art dual column, nitrogen producing air separation unit against the embodiments shown in FIGS. 1-3 and described herein are shown in Table 1).

Admittedly, each of the proposed embodiments of the present dual column, nitrogen producing air separation unit require an increase in capital costs compared to the prior art air separation units as a result of using three condenser-reboilers and the recycle compressor (See FIGS. 1-3), as well as the optional boosted loaded turbine (See FIG. 3), and increase size of the pre-purifier unit (See FIG. 1). In addition, the proposed embodiments operate at slightly higher pressures which must be factored into the designs and costs associated therewith. Like many cryogenic air separation unit designs, the design trade-offs to be considered is whether the additional nitrogen recovery and reduce power costs of the disclosed embodiments outweigh the additional capital costs and higher pressure requirements for the components within air separation unit.

TABLE 1 Prior Art Dual Column Dual Column Dual Column Dual Column Nitrogen ASU Nitrogen ASU Nitrogen ASU Nitrogen ASU of FIG. 1 of FIG. 2 of FIG. 3 Feed Air Flow (Nm³/h) 40385.3 36082.9 37089.2 36089.2 Feed Air Flow Pressure (psia) 139.4 151.9 178 169 Recycle Flow (% of Feed Air) 0.0% 34.6% 25% 35% Recycle Pressure (psia) NA 91.3 115.6 101.1 High P Product Flow (Nm³/h) 11025 15986.7 15126.3 8817.7 High P N2 Pressure (psia) 130 142.5 169 160 Lower P N2 Product Flow (Nm³/h) 14981 9812.9 1006.4 17635.3 Lower P N2 Pressure (psia) 60 61 61.5 61.5 Relative Power Consumption Baseline −5.00% −5.43%   −1.45%   Nitrogen Recovery (% of Feed Air) 64.0% 71.5% 69.8%   73.2%  

In the computer model simulations run, it was also found that the higher recycle flows may increase the oxygen content in the recycle steam which, in turn, increases the oxygen content of the feed air streams sent to the distillation column system. Thus, by controlling the recycle flow rate, the air separation unit operator has the ability to control the oxygen content in the recycle flow so as to optimize performance of the nitrogen producing, dual column air separation unit.

While the present enhancements to a large-scale, dual column nitrogen producing air separation unit has been described with reference to several preferred embodiments, it is understood that numerous additions, changes and omissions can be made without departing from the spirit and scope of the present inventions as set forth in the appended claims. 

What is claimed is:
 1. An air separation unit comprising: a main air compression system configured for receiving a stream of incoming feed air and producing a compressed air stream; an adsorption based pre-purifier unit configured for removing impurities from the compressed air stream and producing a compressed, purified air stream; a main heat exchange system configured to cool the compressed and purified air stream to temperatures suitable for fractional distillation; a distillation column system comprises a higher pressure column and a lower pressure column linked in a heat transfer relationship via a first condenser-reboiler; wherein the higher pressure column is configured to receive the cooled, compressed, purified air stream and produce a nitrogen enriched overhead and an oxygen-enriched kettle stream; wherein the lower pressure column is configured and produce a lower pressure nitrogen product stream, an overhead stream and an oxygen-enriched bottoms; wherein the first condenser-reboiler is configured to condense a first portion of the nitrogen enriched overhead from the higher pressure column against the oxygen-enriched bottoms from the lower pressure column to produce a nitrogen reflux stream for the higher pressure column and an ascending vapor stream in the lower pressure column from the boil-off of the oxygen-enriched bottoms; wherein the distillation column system further comprises a second condenser-reboiler operatively associated with the higher pressure column and configured to condense a second portion of the nitrogen enriched overhead from the higher pressure column against a first split portion of the oxygen-enriched kettle stream from the higher pressure column to produce a liquid nitrogen stream and a recycle stream from the boil-off of the oxygen-enriched kettle stream; wherein a second split portion of the oxygen-enriched kettle stream is introduced into the lower pressure column at an intermediate location; wherein a third portion of the nitrogen enriched overhead from the higher pressure column is taken as a medium/high nitrogen product stream; wherein the distillation column system further comprises a third condenser-reboiler operatively associated with the lower pressure column and configured to condense the nitrogen overhead from the lower pressure column against the oxygen bottoms from the lower pressure column to produce a nitrogen reflux stream for the lower pressure column and a waste stream; and wherein the recycle stream is recycled to: (i) the main air compression system and combined with the incoming feed air stream; (ii) a location upstream of the main heat exchange system and combined with the compressed, pre-purified air stream; or (iii) to the main heat exchanger system.
 2. The air separation unit of claim 1 further comprising an upper column turbine circuit having a booster compressor and a turbine; and wherein a portion of the compressed, purified air stream is diverted to the upper column turbine circuit as a turbine air stream and the booster compressor is configured for further compressing the turbine air stream; turbine air stream main heat exchange system is further configured for partially cooling the further compressed turbine air stream; and wherein the turbine is configured to expand the partially cooled turbine air stream to produce an exhaust stream that is introduced into the lower pressure column
 3. The air separation unit of claim 1 further comprising: a recycle compressor configured to compress the recycle stream and direct the compressed recycle stream to an inter-stage location of the main air compression system; and the main heat exchange system is configured to cool the compressed, purified air in part via indirect heat exchange with the recycle stream.
 4. The air separation unit of claim 1 further comprising: a recycle compressor configured to compress the recycle stream and direct the compressed recycle stream the location upstream of the main heat exchange system where the compressed recycle stream is combined with the compressed, pre-purified air stream; and the main heat exchange system is configured to cool the compressed, purified air in part via indirect heat exchange with the recycle stream.
 5. The air separation unit of claim 1 further comprising: a recycle compressor configured to compress the recycle stream and direct the compressed recycle stream to the main heat exchange system where the compressed recycle stream is further cooled; and wherein the further cooled recycle stream is introduced into the higher pressure column of the distillation column system.
 6. The air separation unit of claim 5 wherein the recycle compressor is a cold compressor driven by a booster loaded turbine and wherein the booster loaded turbine is configured to expand a diverted portion of the medium/high nitrogen product stream to produce an exhaust stream from the booster loaded turbine that is combined with the lower pressure nitrogen product stream.
 7. The air separation unit of claim 1 wherein the liquid nitrogen stream is reintroduced to the higher pressure column with the nitrogen reflux stream for the higher pressure column. 